CVE-2026-72330: net/tls: Consume empty data records in tls_sw_read_sock()

In the Linux kernel, the following vulnerability has been resolved:

net/tls: Consume empty data records in tlsswreadsock()

A peer may send a zero-length TLS applicationdata record; TLS 1.3 explicitly permits these as a traffic-analysis countermeasure (RFC 8446, Section 5.1). After decryption such a record has fulllen == 0. tlsswreadsock() hands it to the readactor, which has no payload to consume and returns zero. The loop treats a zero return as backpressure (used <= 0), requeues the skb at the head of rxlist, and stops. rxlist is serviced head-first on the next call, so the empty record is dequeued, fails the same way, and is requeued again; every later record on the connection is blocked behind it.

tlsswrecvmsg() does not stall on this: a zero-length data record copies nothing and falls through to consumeskb(). Mirror that in the readsock() path by recognizing an empty data record before the actor runs, consuming it, and continuing.